Physics and Engineering of Radiation Detection

1.6. General Properties and Sources of Particles and Waves 41

1.6.B Electrons

According to our understanding so far, electron is one of the fundamental particles
of nature. It carries negative electrical charge and has very small mass and radius.
Although we talk in terms of radius, none of the experiments so far has been able to
associate any particular structure to it. Interestingly enough, even though it appears
to have no structure, it seems to be spinning in well defined ways.

Basic Properties of Electrons

Rest mass = 9. 11 × 10 −^31 kg=0. 511 MeV/c^2

Electrical charge = − 1. 602 × 10 −^19 C

Internal structure : Believed to have no internal structure.

Electrons were first discovered by J. J. Thompson in 1897 in Britain, about 6
years after their presence was hypothesized and they were namedelectronsby an
Irish physicist, George Stoney. Thompson was able to produce cathode rays (called
as such because the rays seemed to be originating from the cathode) by making
an electric current pass through a glass bulb containing little amount of air. This
produced light of different colors inside the glass bulb and also a faint spot on the
wall of the bulb. He managed to change the direction of these rays by applying
an electric and a magnetic field across the bulb such that the electric field would
move the rays in one direction and the magnetic field would move the rays in the
other. With this scheme he was able to find the charge to mass ratio (e/m) of the
particles in these rays. Interestingly enough the mass of this particle was found
to be hundreds of times smaller than the atom. This was the first discovery of a
subatomic particle.
Later on, Ernest Rutherford proposed that the atom actually contained a nucleus
where the positive charges were concentrated. Neil Bohr expanded on this idea by
assuming that electrons revolved around this nucleus in well defined orbits, a picture
that we now know to be correct.
Just like light waves that sometimes behave as particles, electrons also seem to
have wave-like properties. For example when one atom comes closer to another, their
electron clouds (electrons revolve around the nucleus in a cloud-like orbit such that
they appear to be everywhere at the same time) interfere and may form a molecule.
Interference of electron waves has also been observed in double slit experiments when
electrons were forced to pass through consecutively placed slits.
Our familiar electric current is carried through the metallic wires by electrons that
drift at the application of electric potential at the ends of the conductor. But this is
not all, by adding or removing electrons from some materials, called semiconductors,
scientists have been able to fine tune and control their conduction properties. This
has enabled them to construct sophisticated electronic components that now form
the backbone of our technological development.
Electrons are also extensively used in medical diagnostics, therapy, material re-
search and a number of other fields.